I have this question to do for my next class:

A gutter is to be made from a metal sheet of width 3 metres by bending up one third of the sheet on each side through an angle θ. How should the θ be chosen so that the gutter will carry the maximum amount of water?

This is what I pictured it to be:

However, after that, I can't figure out where to start.

Could someone help me out? A full solution would be very much appreciated.